Wave solder machines are particularly useful to solder expeditiously plural electrical connections on a printed circuit board or the like, as is well known. Typically the circuit board with various components inserted therein is conveyed automatically via fluxing, pre-heating, and/or like pretreating stations ultimately to engage a continuously replenished wave-like formation of molten solder commonly called a solder wave. As the circuit board is conveyed through the solder wave, soldered connections are made. Liquid wave machines also are used for tinning and other purposes. The invention will be described below with reference to a wave solder machine, but it will be understood that the features of the invention may relate equivalently to other liquid wave machines for coating and the like.
A so-called fountain or generator produces the solder wave. The fountain typically is positioned in a main supply tank containing a recirculating supply of molten solder that is pumped through the fountain into the wave and returns from the latter and back into the recirculating solder supply. Propeller and impeller type pumps have been used to pump the solder through the fountain to form the solder wave.
The opening at the top of the fountain is positioned such that its elongated width dimension is transverse to the direction of movement of the printed circuit boards, and the relatively shorter length dimension of such opening at the ends thereof is in a direction approximately parallel to the conveyed direction of the printed circuit board. The relatively narrow ends of the fountain establish side boundaries for the solder wave; the width dimension of the solder wave is considered that dimension between such side boundaries. The fountain may be designed to have the solder wave flow over one or both of the elongate fountain sides; in either case the length dimension of the solder wave is that dimension thereof taken in a direction approximately parallel to the conveyed direction of the printed circuit boards, i.e. parallel to the fountain ends. The height or depth dimension of the solder wave is the height thereof or depth of material thereof above the top of the fountain.
To optimize the soldered connections effected by the wave soldering machine, it is desirable that the solder wave be substantially non-turbulent (wave turbulence increases dross, impurities in the solder) and that the distribution of solder in the solder wave, e.g. the height and contour, be substantially uniform, i.e. the cross-sectional profile of the solder wave looking in a direction transverse to the length thereof should remain substantially uniform in height especially as well as in contour, over the entire width of the solder wave.
Due to the relatively heavy mass or specific gravity of molten solder it has been difficult in the past to redict a horizontally flowing stream of solder to a vertical flow direction in a fountain and still to obtain a uniform distribution of solder in the wave. The flow momentum of the horizontal solder stream would tend to cause the downstream end of the wave to have more solder than the upstream end. Therefore, one or more screens, baffles, tortuous flow paths, and other flow impedances have been used in prior fountains to improve wave uniformity in the solder wave. However, unnecessary energy is consumed to pump the solder through those flow impedances.
The unnecessary consumption of energy to pump and to heat solder in a wave having a width appreciably wider than that of the printed circuit board carried therethrough also is a disadvantage of prior wave soldering machines. In such machines the conveyor may be adjusted or adapted to carry printed circuit boards of different sizes, but the width of the solder wave remains constant so that only a portion thereof actually is used to effect a soldering function when relatively narrow printed circuit boards are soldered.
In prior wave soldering machines the solder flow returning from the wave over the elongate edge of the fountain drops essentially vertically directly into the supply of molten solder contained in the main supply tank. Such vertical dropping effects turbulence in the solder supply causing dross to easily be drawn in by the fountain pump and delivered into the wave. Impingement of dross on the circuit board may cause an undesirable high impedance connection, for example. Accordingly, it is desirable to confine the dross, which normally has a smaller specific gravity than the solder, to the surface of the recirculating supply of solder thereby maintaining the purity of the solder flowing in the solder wave.